net: ethernet: mtk_eth_soc: initialize PPE per-tag-layer MTU registers

The PPE enforces output frame size limits via per-tag-layer VLAN_MTU
registers that the driver never initializes. The hardware defaults do
not account for PPPoE overhead, causing the PPE to punt encapsulated
frames back to the CPU instead of forwarding them.

Initialize the registers at PPE start and on MTU changes using the
maximum GMAC MTU. This is a conservative approximation -- the actual
per-PPE requirement depends on egress path, but using the global
maximum ensures the limits are never too small.

Fixes: ba37b7caf1ed2 ("net: ethernet: mtk_eth_soc: add support for initializing the PPE")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Link: https://patch.msgid.link/ec995ab8ce8be423267a1cc093147a74d2eb9d82.1775789829.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>